The present invention relates to combustion systems and more particularly to a self-checking safety switch control circuit for a power combustion sytem. Specifically, the present invention relates to a self-checking safety switch control circuit for preventing operation of the fuel supply system of a power combustion system when a flow detector indicates that there is insufficient flow of combustion air and products of combustion through the combustion system. The circuit is self-checking because it prevents the initiation of combustion in the system if the flow detector has not returned to its pre-combustion operating condition from its condition when the system is operating normally.
Power combustion systems are those which mechanically assist the flow of combustion air and products of combustion through the combustion system. This is normally accomplished by the use of a fan or blower to either push the combustion air into the combustion chamber of the combustion system under positive pressure or to draw the products of combustion out of the combustion chamber by negative pressure. No matter how the mechanical assist is provided, almost all power combustion systems require proper operation of the assist means for combustion to proceed in a safe manner. As a result, power combustion systems generally employ some method of proving the operation of the assist means. Sail switches, pressure switches and centrifugal switches, among others, have been used for this purpose. These devices typically employ a single electrical contact which closes when proper operation of the assist means is established. This has the inherent disadvantage that if the electrical contacts weld together or if the actuating mechanism sticks or binds, the system indicates that flow is adequate even if it is not. Further, when such a failure occurs, there is no indication that the proving system is disabled.